1. Field of the Invention
The present invention relates to a pixel structure, a thin film transistor (TFT), and a method of fabricating the same, and more particularly, to a pixel structure using the array on color filter (AOC) technique and a method of fabricating the same.
2. Description of the Related Art
A Thin Film Transistor Liquid Crystal Display (TFT LCD) is generally composed of a TFT array substrate, a color filter array substrate, and a liquid crystal layer between two substrates. In addition, in an alternative method of fabricating LCDs, the TFT array is fabricated on the color filter array, called an array on color filter (AOC) technique. In other words, in such method, a color filter array is formed over the substrate, and the TFT array is formed over the color filter array later.
In general, the TFT array substrate comprises a plurality of pixel structures, wherein each pixel comprises a TFT and a pixel electrode that is typically formed by a five-photomask process. Wherein, in the first photomask process, a first metal layer is defined for forming components such as a scanning line and a gate of the TFT. In the second photomask process, a channel layer of the TFT and an ohmic contact layer are defined. In the third photomask process, a second metal layer is defined for forming components such as a data line and a source/drain of the TFT. In the fourth photomask process, a passivation layer is patterned. In the fifth photomask process, a transparent conductive layer is patterned for forming a pixel electrode.
However, following the trend of larger-sized TFT LCDs, different kinds of problems and challenges, such as lowering yield rate and decreasing production capacity, are inevitable. Therefore, if the number of photomasks during the TFT fabricating process can be reduced, or the steps of exposure process in fabricating the TFT components can be reduced, the fabricating time can be reduced, the production capacity can be improved, and the manufacturing cost can be lowered.
A four-photomask process had been introduced and commonly used in manufacturing pixel structures, wherein one of the photomasks having halftone patterns is used, to reduce one photomask. However, for the photomask with halftone patterns, the photomask layout is harder to design and the choice of photoresist may be insufficient. In addition, in photomasks with halftone pattern, the photoresist patterns after exposure are usually not ideally uniform.